Method of increasing the fatigue resistance and creep resistance of metals and metal body formed thereby

ABSTRACT

THE FATIGUE AND CREEP RESISTANCE OF A METAL BODY IS SUBSTANTIALLY INCREASED BY FIRST PRESTRESSING THE METAL BODY IN NON-COLD WORKED CONDITION AT OR BELOW THE PROPORTIONAL LIMIT BUT AT A STRESS VALUE WHICH IS GREATER THAN THE STRESS VALUE TO WHICH THE METAL BODY IS TO BE SUBJECTED DURING NORMAL USAGE, AND THEN EITHER RELAXING THE SURFACE LAYER OF THE BODY OR REDUCING THE THICKNESS OF THE BODY BY A PREDETERMINED AMOUNT.

l. R. KRAMER June 18, 1974 METHOD OF INCREASING THE FATIGUE RESISTANCE AND CREEP RESISTANCE 0F METALS AND METAL BODY FORMED THEREBY 4 Sheets-Sheet 1.

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METHOD OF INCREASING THE FATIGUE RESISTANCE AND CREEP RESISTANCE 0F METALS AND METAL BODY FORMED THEREBY Filed March 9, 1972 4 Sheets-Sheet 3 \O' CREEP RATE 83 (MINUTE I69 I I UNTREATED PRESTRESSED IOOKSI, suREAcE LAYER REMOVED PRESTRESSED \OOKSI RELAX, N0 SURFACE LAYER REMOVED PRESTRESS 65KSl-550F RELAX AT 550E,MO SURFACE LAYER REMOVED PRESTRESS 70KS|-550F RELAX AT 550F, N0 SURFACE LAYER REMOVED LEGEND IS SSEIHlS CREEP RESISTANCE 0F TITANIUM (6 AL- 4V) (TEST TEMPERATURE June 18, 1974 '1. R. KRAMER 3,317,796

METHOD OF INCREASING THE FATIGUE RESISTANCE AND CREE]? RESISTANCE OF METALS AND METAL BODY FORMED THEREBY ISM) 883818 I. R. KRAMER June 18, 1974- CREE? RESISTANCE OF METALS AND METAL BODY FORMED THEREBY 4 Sheets-Sheet 4.

Filed March 9, 1972 80mm 2 m: 2 $3 56m $3555:

Q moomw 6 m: m; $3 $3 Ewwmfiwmmm I Fog E m: m 52 5502mm wufi w @Ew omwmmsmwmm D 9 302% 524 @0556 6x8 mmmEmmEmfl x u v 206572052: 7: 35m: 3502: 3,155 02 56w mmw wmmm o m v 23253 2 8322 o E: H 525: wwmEmmEm oz 0 Q 550 2 3 5: 6 E5232 NH: 00- 02 V0- mm AW 3 u [8X] ssaais United States Patent Oflice 3,317,796 Patented June 18, 1974 3,817,796 METHOD OF INCREASING THE FATIGUE RE- SISTANCE AND CREEP RESISTANCE OF METALS AND METAL BODY FORMED THEREBY Irvin R. Kramer, Denver, Colo., assignor to Martin Marietta Corporation, New York, N.Y. Continuation-impart of abandoned application Ser. No. 51,232, June 30, 1970. This application Mar. 9, 1972, Ser. No. 233,183

Int. Cl. C22f 1/04, 1/18 U.S. Cl. 148-115 R 16 Claims ABSTRACT OF THE DISCLOSURE The fatigue and creep resistance of a metal body is substantially increased by first prestressing the metal body in non-cold worked condition at or below the proportional limit but at a stress value which is greater than the stress value to which the metal body is to be subjected during normal usage, and then either relaxing the surface layer of the body or reducing the thickness of the body by a predetermined amount.

This application is a continuation in part application of application Ser. No. 51,232 filed on June 30, 1970, by applicant, and identically entitled and now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to increasing fatigue and creep resistance of metal bodies whose endurance limit is below the proportional limit and, more particularly, to such a method for treatment of metal bodies of aluminum, and titanium alloys in non-cold worked condition.

Description of the prior art The fatigue life of metal bodies have, in the past, been increased by work hardening of the metal by shot peening. However, shot peening does not work on all metals and is quite diflicult to control.

Through research, it has been discovered that when the metals are deformed in the plastic range, even uniaxially, the work hardening of the metal is not uniform throughout the cross section of the specimen or metal body. Instead, a layer which extends approximately 0.004 I inch from the surface, work hardens to an extent substantially greater than the bulk. This surface layer contains a larger concentration of defects, such as dislocations, vacancies, dipoles, etc. than the bulk material. Further, it has been discovered that the surface layer can relax with time. The relaxation of the surface layer contributes to the creep strain. Discovery has also been made that the presence of this surface layer decreases the fatigue resistance of metal bodies.

SUMMARY OF THE INVENTION .Since the presence of a highly work hardened surface layer decreases the fatigue life and creep resistance of metals, it is necessary to insure that during the fatigue and/ or creep process occurring with use, this surface layer of the metal body is either materially reduced or completely absent therefrom. The present invention is directed to first prestressing (prestraining) the metal body in noncold worked condition at or below the proportional limit to a stress (strain) greater than that experienced by the body during the fatigue and/or creep process occurring under normal use. Subsequent to prestressing the metal body, the surface layer formed during the prestressing of the metal body is then eliminated. When the specimen is deformed at a stress near the proportional limit, all soft grains will be deformed plastically. Then the surface layer associated with the soft grains is eliminated either by actual removal, say by chemical means or by a low temperature annealing or relaxation treatment. Then, upon subsequent application of loads always less than the prestress load, the surface layer is not reformed, or, if it is reformed, much less is reformed than if the specimen had been loaded or stressed only up to its use stress initially. For metal specimens of /8 inch in thickness, the surface layer depth ranges from 0.002 to 0.005 inch. The surface layer may be removed by mechanical abrasion, chemical, electro-chemical means or by annealing only the surface layer of the metal specimens.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph of fatigue life tested in tension-compression for 6Al-4V titanium specimens untreated, and fully treated under the method of the present invention.

FIG. 2 is a graph of creep rate at 550 F. as a function of stress for 6Al-4V titanium specimens.

FIG. 3 is a graph of fatigue life for specimens of 7075-T6 aluminum with and without treatment under the method of the present invention, as tested in tensioncompression.

FIG. 4 is a graph of fatigue life for specimens of 7075- T6 aluminum with and without treatment under the method of the present invention, as tested in tensiontension.

DESCRIPTION OF THE PREFERRED EMBODIMENT Metal bodies in non-cold worked condition may have their fatigue life and creep resistance increased by being subjected to a process such that the surface layer is not present or has minimum strength during its use. The surface layer which is further hardened during the fatigue process contributes to the fatigue failures. The relaxation of the surface layer with time at temperature decreases the resistance to plastic deformation and therefore increases the creep rate. The surface layer is materially decreased or eliminated by first prestressing (prestraining) the metal body in non-cold worked condition to a stress below the proportional limit but to a stress greater than that which will occur during normal use. During prestressing, a surface layer will form in the grains which undergoes plasic deformation. 'Ihis surface layer is eliminated by low temperature annealing or by removing approximately 0.004 inch from each surface of the specimen, the amount of metal removed being only that required to remove the surface layer. The elimination of the surface layer by relaxation depends upon the particular alloy. In those materials, such as 2014-T6 and 7077- T6 aluminum alloys, the dislocations are so strongly pinned that the surface layer can be eliminated by relaxation without also affecting the interior. In those cases where the surface layer can relax at the temperature of use, the presence of the surface layer is undesirable and its elimination increases the creep resistance. In contrast, for those alloys the surface layer is stable at the temperature of use, the elimination of the surface layer does not affect the creep behavior. In either case, elimination of the surface layer increases the resistance to crack propagation and fatigue. In general, the surface layer depth resulting from prestressing ranges from 0.002 to 0.005 inch for A; inch specimens. However, this depth decreases with increasing section size.

For different metals whose bodies vary in size, the layer depth will necessarily vary, requiring more or less surface removal, if the step is employed to insure against excessive surface layer build up during subsequent use. The correct thickness of removed material can be ascertained only from a trial and error basis for each given metal or alloy and for each given body size thereof.

Although the present invention has been described in conjunction with the theory, believed to occur, that during prestressing of a metal body in non-cold worked condition at or below the proportional limit with the immediate surface layer thereof becoming work hardened to an appreciably greater extent than the bulk of the body and that the invention involves subsequently removing the excess work hardening from the surface layer of the same, it is not intended that this invention be limited to this theory.

Regardless of this stated theory, the treatment of the metal body in non-cold worked condition to these two succinct steps has the effect of greatly increasing fatigue and creep resistance, stress corrosion cracking resistance and crack propagation resistance.

EXAMPLE 1 A preferred example of the method in accordance with the present invention for improving the fatigue resistance of a titanium metal body is as follows: the block or rod of 6Al-4V titanium in non-cold worked condition was subjected to a tensile stress (prestressed) of 100,000 p.s.i. The titanium specimen was A; inch in diameter and 3.75 inches long. A work hardened outer layer of approximately 0.004 inch was formed which was completely removed electrochemically.

The result of treating the titanium specimen in accordance with the process of the present invention is illustrated in FIG. 1 of the drawings. The improvement in the fatigue life of the titanium specimen, by prestressing of the same and removing of the surface layer, may be readily appreciated from the test results shown. The specimens were tested in tension-compression by a conventional fatigue testing process. The fatigue curve identified at A illustrates one of the test results for untreated specimens of 6Al-4V titanium while curve B illustrates the test results for fully treated specimens of like metal. Referring to curve B, specimens identical to that of curve A were prestressed to 100,00 p.s.i. by exerting tension on the same followed by surface layer removal. A noted improvement in the fatigue life of the titanium specimens occurred. The endurance limit was raised from 76,000 p.s.i. to 84,000 p.s.i. At 85,000 p.s.i. the life was increased from 50,000 cycles of 80,000 cycles.

The effect of stressing and surface removal (SR) treatment on the creep behavior of the titanium (6Al-4V) specimens is readily seen from the following table:

TABLE 1 [Creep behavior of Titanium (6Al/4V specimens] Percent re- Creep rate 0. duction in (minute the transient Stress creep strain Temp. F.) (k.S.1-) Lntreated SR treated in 20 hrs.

70 1. 6X10- 4. 6X10-' 36 60 5. 7 10- 3. 9X10- 57 65 6.3)(10- 5.1X10- 54 70 7. X10- 6. 0X10- 25 50 4. 6X10 1. 4X10 Best results were obtained by prestressing the specimens at 100 k.s.i. At 550 F., the steady state creep was re duced from 1.6x minuteto 4.6 10 minute-- by prestressing and surface removal. The steady state creep rate for untreated and SR treated specimens at 600 F. is shown in FIG. 2. It is revealed in FIG. 2 and Table 1 that the steady state creep rate for SR treated specimens is much lower than the untreated specimens.

Prestressing may occur at room temperature or elevated temperature with the results being approximately the same insofar as the creep resistance is concerned. It is noted in FIG. 2 that regardless of whether the specimen in noncold worked condition was prestressed alone, prestressed and relaxed at room temperature, prestressed and surface removed or prestressed to either 65 k.s.i. or 70 k.s.i. at 550 F. and relaxed at the same temperature, and re gardless of whether there was surface removal or not, the test points fell on approximately a common line. Thus, it is definitely the prestressing at values above the stress values occurring during normal use but at or below the proportional limit that advantageously afiects the metal bodies.

EXAMPLE 2 The improvement in the fatigue life of aluminum 7075- T6 may be readily seen by reference to FIG. 3. In this case, the aluminum alloy specimens were /8 inch in diameter and of a length of 3.75 inches and subjected to a tensile prestress of 50,000 p.s.i. with a layer of 0.004 inch subsequently removed from the surface. Removal of the surface layer was achieved again in an electrochemical process involving an electrochemical solution of wherein the specimen constituted the anode and a current density of about 2 amps per square inch was employed.

Reference to FIG. 3 shows curve A in the plot of fatigue, wherein the endurance limit of the untreated specimens is approximately 23,000 p.s.i. at 10 cycles while, when similar specimens were prestressed to 50,000 p.s.i. and a layer of 0.004 inch was removed from the surface, the endurance limit in the tension-compression fatigue test increased appreciably from 23,000 p.s.i. to 34,000 p.s.i. as seen by curve B.

Similarly, by reference to FIG. 4, the improvement in fatigue life of similar aluminum 7075-T6 specimens is shown for such specimens fatigue tested in tension-tension and treated in accordance with the present invention. Curve A in the middle illustrates the base line fatigue life of untreated specimens. Curve B illustrates the fatigue life for specimens that were prestressed to 60,000 p.s.i. in tension, while curve C shows the fatigue life of specimens that were prestressed to 60,000 p.s.i., similarly in tension, with the surface layer removed under the method of the present application. Again, a layer of 0.004 inch was removed from the surface subsequent to the prestress treatment. Compared to the base line data, curve A, the prestress and surface removal treatment improves the fatigue resistance, as illustrated in curve C, while prestressing alone decreases the fatigue life (curve B). Additional data in the form of aging subsequent to prestressing and surface removal or prestressing alone illustrates a further improvement in the fatigue life regardless of aging before or after the surface removal treatment. Illustrated by the square points, specimens that were prestressed at 65,000 p.s.i. and aged at 250 F. for 1.5 hours after removal of the surface layer had a fatigue life at the same test stress greater than that indicated by the data presented in curves B and C.

Under the method of the present invention, especially as employed with the 7075-T6 aluminum alloy bodies, the prestress level is critical. Specimens prestressed to values above 65,000 p.s.i. and tested in tension-compression did not show any improvement in the fatigue life of the same, compared to the base line curve, when the surface layer was removed. Similar results were obtained for specimens tested in tension-tension when the specimens were prestressed above 65,000 p.s.i. before removing the surface layer. For example, prestressing at 70,000 p.s.i. followed by the surface removal treatment, did not improve the fatigue resistance of the test body.

While the present invention has been described in conjunction with metals comprising titanium, and aluminum alloys, the invention has application to structural metals in general including iron. The multiple step process of the present invention also increases the creep resistance of the metal body. For instance, in prestressing the titanium alloy (6Al/4V), to 90,000 p.s.i. and removing approximately 0.005 inch from each surface and then testing the metal body for creep resistance at 80,000 p.s.i. at

550 F., the creep rate decreased by a factor of about 10. It is believed that for many metal bodies, sophisticated fixturing is not required and for built-up structures, the effect may be accomplished by prestressing the structure itself under the same conditions under which it will be used. Further, chemical or electrochemical milling isenot always required. For example, for titanium alloys, prestressing followed by a room temperature rest period for a few hours results in a natural elimination of the excess work hardening of the outer surface by annealing. Further, the treatment of the present invention does not necessarily have to be conducted on finished assemblies but may be conducted on the component parts which, in themselves, may be subjected to failure by fatigue or creep, etc.

, It is important to note that in the present method of treating the metal bodies, the cross-sectional area is not reduced substantially under the treatment since, at most, an. extremely thin outer layer of material is removed which does not appreciably modify the dimensions of the metal body being treated.

There are many ways to remove the surface layer. In principle, the method must be such that the work-hardened surface layer is not reformed by the removal method. Chem milling may be used, or electrochemical removal and low temperature annealing in those cases when the surface layer is sufficiently unstable. It was found that the surface layer may be eliminated in low alloy aluminum and titanium alloys by annealing at room temperature. In annealing, care must be taken not to anneal (soften) the interior of the specimen. One can determine whether or not the interior is affected by the annealing treatment by examining the stress-strain behavior after the annealing treatment. When the annealing treatment affects only the surface layer of the strained specimen, the stress-strain curve always joins that of the original specimen. If the interior is softened by the annealing treatment, the stressstrain will fall below that of the original specimen.

In addition to the above methods of eliminating the surface layer, it appears that very light machining or honing or grinding or spark machining may be used. In these cases, care must be taken not to cold work the metal.

The method for prestressing may be varied, and although the methods described here mainly involve axial tension and compression, it is the level of stress that is important. Bending, torsion, and very light burnishing may be employed. The prestress limit is up to, but not including, the proportional limit. The proportional limit is defined as that stress -where the stress-strain must first become non-linear. This value depends upon the sensitivity of the strain measurements.

The method of the present invention has application to metal bodies which, as purchased from a vendor or otherwise obtained, are not in a cold-worked condition. Further, with the exception that the present invention uses a low temperature annealing treatment, the metal body is in its final heat treat condition, i.e., annealed, normalized, quenched and tempered (steel) or solution treated and aged (aluminum and titanium). In all cases, it is only important that the metal body prior to treatment by the present process be in non-cold worked condition, wherein the metal body has not been stressed above the proportional limit. Such a metal body is then subjected to the prestress at slightly below the proportional limit but at a stress value which is greater than the stress value to which the body is to be subjected during normal usage and then the thickness of the body is reduced or the body is relaxed by low temperature annealing to substantially eliminate the excess work hardened outer surface. Where such a metal body is received from the vendor, for instance, in cold worked condition, the body must be subjected to a process to remove the effects of cold working,

such as annealing or stress relieving. Then the body is processed the same as above.

What is claimed is:

1. A method of treating a non-cold worked metal body comprising in sequence the steps of:

prestressing the metal body in non-cold worked condition, including its interior, at or below the proportional limit but at a stress value which is greater than the stress value to which the metal body is to be subjected during normal usage, and

low temperature annealing the surface layer of said body without annealing the interior of said body.

2. A method of treating a solution treated, cold worked and aged metal body comprising the steps of:

prestressing the metal body in non-cold worked condition, including its interior, at or below the proportional limit but at a stress value which is greater than the stress to which the metal body is to be subjected during normal usage, and

reducing the thickness of the body by a predetermined amount.

3. A method of treating a cold worked metal body comprising in sequence the steps of:

removing the effects of the cold working within said body,

prestressing the metal body, including its interior, at

or below the proportional limit, but at a stress value which is greater than the stress to which the metal is to be subjected during normal usage, and

low temperature annealing the surface layer of said body without annealing the interior of said body.

4. A method of treatinga metal body in non-cold worked condition, comprising in sequence the steps of:

prestressing the metal body, including its interior, at

or below the proportional limit but at a stress value which is greater than the stress value to which the metal body is to be subjected during normal usage, and

reducing the thickness of the body by a predetermined amount.

5. A method of increasing fatigue and creep resistance of a metal body comprising in sequence the steps of:

prestressing the metal body in non-cold worked condition, including its interior, at or below its proportional limit but at a stress value which is greater than the stress to which the metal body is to be subjected during normal usage to form a surface layer which is work hardened to an appreciably greater extent than the bulk of said body, and removing the work hardening of said surface layer only.

6. The method as claimed in claim 5, wherein the excess work hardening of said surface layer is removed by physically removing said surface layer from said metal body.

7. The method as claimed in claim 6, wherein the sur face layer which is removed subsequent to prestressing is on the order of 0.002 inch to 0.010 inch.

8. The method as claimed in claim 5, wherein said metal body comprises one material of the group consisting of aluminum alloys and titanium alloys of the nonprecipitation hardened type.

9. The method as claimed in claim 8, wherein said body is an aluminum alloy metal body which is prestressed at room temperature to values above 25,000 p.s.i. but less than 65,000 p.s.i.

10. The method as claimed inclaim 8, wherein said body is a titanium alloy metal comprising 6% aluminum and 4% vanadium by weight which is prestressed at room temperature to values above 70,000 p.s.i. but less than 100,000 p.s.i.

11. The method as claimed in claim 5, for an age hardening metal body further comprising the step of aging said body, either before or after removing the excess work hardening of the surface layer.

12. The method as claimed in claim 5, wherein said metal body comprises an age hardening aluminum alloy and said method further comprises the step of aging said.

metal body at a temperature in the range of 100 F. to 300 F. for approximately 1 to 5 hours.

13. The method of increasing fatigue and creep resistance of an age hardened aluminum alloy metal body comprising in sequence the steps of:

prestressing the metal body in non-cold Worked condition, including its interior, at or below its proportional limit until the surface layer thereof is work hardened to an appreciably greater extent than the bulk of said body,

removing the work hardening of said surface layer only,

and

age hardening said metal body at temperatures slightly above room temperature for a time period in excess of one hour.

14. The method as claimed in claim 13, wherein said metal body is aged at 250 F. for approximately 1.5 hours.

15. A metal body having increased fatigue and creep resistance formed by pre-stressing the same in a non-cold worked state, including its interior, at or below the proportional limit but at a stress value which is greater than the stress value to which said metal body is to be subject during normal usage and reducing the thickness of said body by predetermined amount.

16. A metal body having increased fatigue and creep resistance formed by pre-stressing in non-cold worked condition, including its interior, at or below the proportional limit but at a stress value which is greater than the stress value to which the metal body is to be subjected during normal usage and low temperature annealing the same.

References Cited UNITED STATES PATENTS 3,516,874 6/1970 Maker et al. 1484 2,083,576 6/1937 Nock, Jr. 148-12] 2,470,791 5/1949 Reich 148-12.7 3,133,839 5/1964 Thomas 14812.7 3,511,622 5/ 1979 Nation 29193 WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 

